Aperture effects on star formation rate, metallicity, and reddening

Abstract

We use 101 galaxies selected from the Nearby Field Galaxy Survey to investigate the effect of aperture size on the star formation rate, metallicity, and reddening determinations for galaxies. Our sample includes galaxies of all Hubble types except ellipticals with global star formation rates (SFRs) ranging from 0.01 to 100 M⊙ yr -1, metallicities in the range 7.9 ≲ log (O/H) + 12 ≲ 9.0, and reddening of 0 ≲ A(V) ≲ 3.3. We compare the SFR, metallicity, and reddening derived from nuclear spectra to those derived from integrated spectra. For apertures capturing <20% of the B 26 light, the differences between nuclear and global metallicity, extinction, and SFR are substantial. Late-type spiral galaxies show the largest systematic difference (∼0.14 dex), with nuclear metallicities greater than the global metallicities. The Sdm, Im, and Peculiar types have the largest scatter in nuclear/integrated metallicities, indicating a large range in metallicity gradients for these galaxy types, or clumpy metallicity distributions. We find little evidence for systematic differences between nuclear and global extinction estimates for any galaxy type. However, there is significant scatter between the nuclear and integrated extinction estimates for nuclear apertures containing <20% of the B 26 flux. We calculate an "expected" SFR using our nuclear spectra and apply the commonly used aperture correction method. The expected SFR overestimates the global value for early-type spirals, with large scatter for all Hubble types, particularly late types. The differences between the expected and global SFRs probably result from the assumption that the distributions of the emission-line gas and the continuum are identical. The largest scatter (error) in the estimated SFR occurs when the aperture captures <20% of the B 26 emission. We discuss the implications of these results for metallicity-luminosity relations and star formation history studies based on fiber spectra. To reduce systematic and random errors from aperture effects, we recommend selecting samples with fibers that capture >20% of the galaxy light. For the Sloan Digital Sky Survey and the Two-Degree Field Galaxy Redshift Survey, redshifts z > 0.04 and 0.06 are required, respectively, to ensure a covering fraction >20% for galaxy sizes similar to the average size, type, and luminosity observed in our sample. Higher luminosity samples and samples containing many late-type galaxies require a larger minimum redshift to ensure that >20% of the galaxy light is enclosed by the fiber. Show more